A garage damaged my friend's head gasket whilst removing spark plugs during a service - is the garage liable?

A friend took her car for an MoT and service earlier today. The car passed its MoT fine. When they were doing the service, apparently they were removing the spark plugs (which were apparently rusty and should have been done last service) and dropped one. It has damaged the head gasket and now will cost me £800 to get fixed, on top of the other costs. Does it sound right that dropping a spark plug can do so much damage? Is the garage liable for the damage? Will their insurance cover it? It passed its MoT so this confirms it was running ok beforehand. What else can we do?
Yes, if spark plugs are not regularly removed and greased they can fuse to the head and snap when a garage tries to remove them. Find out what the car is worth and judge from that whether it is worth fixing.
Answered by Honest John on

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